This invention relates generally to metal founding, and more particularly to an improved method of and apparatus for casting molten metal in a continuous casting machine of the wheelband type.
In a known continuous casting system, molten metal is flowed into a mold formed by enclosing an arcuate portion of the peripheral groove of a rotatable casting wheel with a flexible endless metal band. As the casting wheel rotates, a coolant is applied to the external surfaces of the wheel adjacent the peripheral groove and to the flexible band to prevent excessive heating thereof and to achieve rapid solidification of the molten metal. Typically, a relativley thick (.060 in. or greater) metal band is positioned about and urged into engagement with the flanges of the groove by one of a number of known arrangements of band tensioning, positioning and guide wheels. Many of these arrangements necessitate the bending or flexing of the band about the periphery of one or more band wheels of significantly smaller diameter than that of the casting wheel. Several such arrangements are shown in U.S. Pat. No. 3,596,705, issued Aug. 3, 1971 to John H. Murphy and assigned to the assignee of this invention.
As explained in the aforementioned U.S. Pat. No. 3,596,705, the continual bending and flexing of the band about the casting and band wheels subjects the band to undesirable strain which is proportional both to band thickness and to the diameter of the band wheels. This strain tends to cause rapid deterioration of the band and resultant failure after only a few hours of operation. It will be appreciated by those skilled in the art that the short life expectancy of the band and the time consuming replacement thereof are major problems concerning the efficient operation and maintence of continuous casting machines.
When continuously casting metal in machines of the above-described type, such as, for example, copper, aluminum and steel, it is highly advantageous to solidify the molten metal in as short a period of time as possible in order to maintain a high casting rate. Moreover, when casting metals containing alloying elements, rapid solidification is desirable in order to maintain intermetallic compounds in solid solution and to limit the size of the particles that do precipitate out of solution. However, the low cooling efficiency and non-uniformity of heat transfer inherent in the thick metal bands of prior art casting machines preclude attainment of the high casting rates and metallurgic effects desired. This is especially so if the band is fabricated of a material having a relatively low rate of heat transfer even though coolant is applied to the external band surface at maximum practicable pressure and volume.
It should be apparent, therefore, that to improve the casting rate of continuous casting machines of the type described concomitantly with the life expectancy of the flexible band element, bands formed of thin gauge materials having a high heat transfer rate could be advantageously utilized to achieve such improvements. However, the prior art recognizes several problems in attempting to fabricate the band from thin guage, high heat transfer materials. Notably, one problem encountered when fabricating mold components of high heat transfer rate materials is the typically low structural strength of such materials which adversely affects their useful life, as explained in U.S. Pat. No. 3,464,483, issued Sept. 2, 1969 to Daniel B. Cofer and assigned to the assignee of the present invention. While reducing band thickness would tend to reduce band strain exerted by the band wheels on a band formed of a high heat transfer rate material, the structural strength of the band would, of course, be further diminished so that little advantage in terms of band life could be realized.
A further problem associated with the use of thin gauge band elements for continuous casting machines is explained in U.S. Pat. No. 3,533,463, issued Oct. 13, 1970 to Robert W. Hazelett et al, wherein the patentees recognize the susceptibility of damage to a thin guage band, particularly at the edges thereof, when it is adequately tensioned to prevent leakage of molten metal from between the band and casting wheel groove. It is further noted in the aforesaid Hazelett et at patent that the band wheel flanges used to steer the band into position for enclosing the groove are especially damaging to the edges of a thin guage band.
One prior art method and apparatus for improving band life and casting rate is described in U.S. Pat. No. 3,642,055, issued Feb. 15, 1972 to William Nighman wherein a foraminous wire mesh belt is employed to close the peripheral groove of the casting wheel of a wheel-band type continuous casting machine. Coolant is directed through the openings in the belt to impinge directly upon the molten metal in the groove. The high surface tension of the molten metal is relied upon to prevent the molten metal from flowing through the openings in the foraminous belt. However, at the high casting rates contemplated by the present invention, the surface tension of the molten metal would very likely be insufficient to prevent leakage through the foraminous belt and the resultant danger of metal splatter in the area surrounding the casting machine. Even if leakage could be prevented, there always exists the possibility of obstructing the pores of the foraminous belt with solidified metal, rendering the escape of the vaporized coolant from the casting groove more difficult and thus increasing the danger of explosion. Moreover, the separation of the cast bar from the foraminous belt as the bar exits the casting groove could be hampered because of adherence between the cast bar and foraminous belt as the molten metal in contact with the belt solidifies. A further disadvantage of the apparatus and method described in U.S. Pat. No. 3,642,055 is the resulting poor quality of the cast bar surface which confronts the coarse and irregular surface of the foraminous belt.